In the present day cryogen cooled and cryogenless MRI magnets, the magnet solenoid mass is suspended in a cryostat with suspension systems that limit thermal losses. Hence, the magnet cold mass temperature is fairly uniform, or at least within the specified temperature gradient to prevent loss of superconductivity of the coils. Therefore, the magnet feet could be mounted without any special thermal designs.
The main concern in designing the magnet mounts for the permanent magnet class of MRI magnets is to maintain static and dynamic mechanical integrity of the magnet in a mobile van environment. In prior designs for mounting permanent magnets in mobile vans, no special thermal mounting design was applied to provide for thermal stability. The magnet was simply mounted on rubber pads which doubled for thermal insulation and vibration isolation material. The main deficiency of this design is that the rubber is compressible. In a mobile MRI trailer van, the magnet must be properly torqued down to the van floor to be able to withstand dynamic loads during transportation of the van. The bolt torque levels are quite high and introduce large compressive load to the rubber in addition to the magnet weight. The rubber's thermal conductance increases with an increase in pressure. This happens for two reasons, (a) as the rubber is compressed, the thermal resistance length is reduced resulting in higher conductance, and (b) the rubber pad often used is synthetic rubber with high percentage of carbon and as the pad is compressed, these carbon atoms thermally short out increasing the thermal conductance of the pad. Consequently, unacceptable temperature gradients were present between the top and bottom pole of the magnet during extreme temperatures outside the mobile van. When the van outside temperature was about or below -29.degree. C., the magnet heaters were not able to keep the magnet temperature at the required magnet imaging temperature specification. This was true even with the van interior, or magnet room, at the required temperature stratification.
It would be desirable then to be able to develop a thermal isolation system for mounting an MRI permanent magnet in a mobile trailer van, while maintaining mechanical integrity.